A retardation film is an optical film utilized for realizing a wider viewing angle or the like of a liquid crystal display. The retardation value of the retardation film depends on wavelength, and the wavelength dispersion of the retardation value is roughly classified into the following three kinds. The first exhibits wavelength dispersion such that a retardation value is larger on the shorter wavelength side (hereinafter referred to as ‘normal dispersion’), the second exhibits wavelength dispersion such that a retardation value scarcely changes ranging from the short wavelength side to the long wavelength side (hereinafter referred to as ‘flat dispersion’), and the third exhibits wavelength dispersion such that a retardation value is smaller on the shorter wavelength side (hereinafter referred to as ‘reverse dispersion’).
Among the three kinds, a retardation film exhibiting reverse dispersion is preferable. The reason therefor is that a retardation film exhibiting reverse dispersion has predetermined retardation (such as λ/2 and λ/4) in a wide wavelength band.
However, generally, many of the retardation films formed from a resin film exhibit normal dispersion.
A retardation film formed from a drawn film of polycarbonate having a fluorene skeleton has been conventionally known (Patent Document 1).
This retardation film develops retardation by a drawing treatment. However, the polycarbonate having a fluorene skeleton has a so high glass transition point that a drawing temperature needs to be determined at an extremely high temperature.
On the other hand, it has been known that a retardation film including polyimide may be formed by coating a solution containing polyimide on a base material and exhibits a predetermined retardation value (Patent Document 2). The formation by coating allows a comparatively thin retardation film to be obtained. However, a retardation film including polyimide generally exhibits normal dispersion (that is, not reverse dispersion).
[Patent Document 1] Japanese Unexamined Patent Publication No. 2002-221622
[Patent Document 2] Japanese Unexamined Patent Publication No. 2000-511296